リフラフォン方式による海上弾性波探査
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概要
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In order to investigate the geophysical property of sea bottom an acoustic exploration method with a sonoprobe, sparker or airgun as a source depending on the investigation purpose has been commonly used. Since this method proviedes a graduation recoding by reflected acoustic wave at boundaries below sea bottom, a vertical section of geological structure can visibly be recognized. However, of the purpose to investigate the hardness of rocks in such cases as an investigation of engineering geology, it is not sufficient because this method does not indicate the wave velocity of rocks. A marine seismic investigation called"Refraphone method"has been carried out by improving seismic source and geophone at sea bottom. In this method 3 to 4 hydrophones (8 to 10 depending on the situation) for the marine refraction method survey are positioned on the profile line at sea bottom and a popping ship navigating on the line at 2 to 3 knots pops an airgun, which in connected to a high pressured nitrogen gas cylinder and kept at a certain depth under the sea surface, at regular interval of 6 to 14 seconds. The refraction waves are recorded in a data recorder simulataneously with the popping timing transmitted from the ship at each receiving point. Some characteristic of this new method are summarized as follows; (A) Since an airgun is used instead of the dynamite, no damage accurs to fishes. There is no explosion sound in the air, as the gun pops more than 5 meters under sea surface. (B) Surveying time can be reduced by an easy operation in comparision with traditional methods. The operation on the sea with a slow tidal current in carried out more efficently than on the ground. This applid also to that in narrow strait with a fast tidal current since it can be carried out taking the advantage of a short period of turning current. (C) A radio wave telrometer used for guiding recording ships and a popping ship makes the position measurment more accurate than before. A refraction wave investigation"Refraphone method"provides similarly reliable data to those obtained from a ground surface seismic investigation. By the analysis of records in"Refraphone method"the survey on the sea surface can easily be performed in order to investigate the distribution of longitudinal wave velocity which indicates the hardness of rocks, velocity and width of fractured zone recognized as a low velocity zone, properties and thickness of weathered layer which covers bed rocks, thickness of sedimentary layer and its velocity which indicates the solidity of the layer, and so forth.
- 1980-03-30